The present invention relates to a solenoid-operated driving apparatus and a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle such as an automobile and which uses a driving apparatus of the above-mentioned type.
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers that are designed so that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc. with a view to improving ride quality and steering stability.
As shown in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 5-149364, by way of example, a conventional damping force control type hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The piston is connected with a hollow piston rod to form a piston assembly. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve. The bypass passage is provided with a damping force control valve for controlling the passage area of the bypass passage.
The damping force control valve is driven by the displacement of a plunger constituting a proportional solenoid driving device provided in the piston rod. When the damping force control valve is actuated to open the bypass passage, the flow resistance to the hydraulic fluid flowing between the two chambers in the cylinder decreases. Consequently, damping force is reduced. When the damping force control valve is operated to close the bypass passage, the flow resistance between the two chambers increases. Consequently, damping force is increased. Thus, damping force can be properly controlled by opening or closing the damping force control valve.
The proportional solenoid driving device used in the damping force control type hydraulic shock absorber disclosed in Publication No. Hei 5-149364 is arranged as follows. The plunger has a main body portion and a magnetically attracted portion smaller in diameter than the main body portion. The attracted portion is connected to the main body portion through a step formed approximately at right angles to the main body portion. The attracted portion is adapted to enter or withdraw from a tubular projection formed integrally with a fixed core. When a coil is energized, a magnetic path is formed in members made of a magnetic material, such as the fixed core and the plunger, thereby attracting the plunger to the fixed core.
The tubular projection of the fixed core is tapered. The plunger is attracted to the fixed core according to the magnitude of the electric current supplied to the coil, which is controlled through a controller. As the plunger approaches the fixed core, the magnetic flux is allowed to escape to the tapered projection in the radial direction, thereby allowing attraction force approximately proportional to the magnitude of the electric current to be obtained.
The above-described prior art suffers, however, from some problems. The attracted portion is formed smaller in diameter than the main body portion because the proportional solenoid driving device is provided in a small space in the piston rod and because the attracted portion of the plunger is caused to enter or withdraw from the tapered tubular projection formed on the fixed core. As shown by the xe2x80x9cprior artxe2x80x9d attraction force characteristic curve in FIG. 3, as the electric current supplied to the coil is gradually increased, in particular, magnetic saturation occurs at the small-diameter attracted portion and so forth, and attraction force becomes insufficient in the latter half of the control zone (i.e., at a position where the plunger has approached the fixed core). It should be noted that the magnitude of attraction force depends on the area of the mutually opposing surfaces of the attracted portion and the fixed core.
Accordingly, there is a plunger displacement region in which the damping force characteristics do not change linearly even if the electric current supplied to the coil is increased at a fixed rate. To solve this problem, it is conceivable to increase the electric current supplied to the coil from the controller only in this displacement region or to provide a spring or the like for assistively urging the plunger in the direction in which it is magnetically attracted. In the former case, however, the consumed current not only increases, but control also becomes complicated, and the load on the controller and the coil increases unfavorably. In the latter case, because a spring or the like for assistively urging the plunger is needed, costs increase correspondingly, and productivity is degraded correspondingly. Thus, these countermeasures involve various problems.
The present invention was made in view of the above-described problems associated with the prior art.
An object of the present invention is to provide a damping force control type hydraulic shock absorber capable of increasing the magnetic force for attracting the plunger in a limited small space with a solenoid-operated driving device having a simplified structure.
The present invention provides a damping force control type hydraulic shock absorber including a cylinder having a hydraulic fluid sealed therein. A piston is slidably provided in the cylinder to divide the inside of the cylinder into an upper chamber and a lower chamber. A hydraulic fluid passage is formed to provide communication between the upper chamber and the lower chamber. A damping force control mechanism is provided in the hydraulic fluid passage to control damping force by controlling the flow of the hydraulic fluid induced in the hydraulic fluid passage by sliding movement of the piston. A solenoid-operated driving device for driving the damping force control mechanism is accommodated in a casing. The solenoid-operated driving device includes a coil provided in the casing. A tubular member is provided in the coil at one end of the coil to form a magnetic path when the coil is energized. A plunger is slidably guided in the tubular member. An attracting member is provided in the coil at the other end of the coil to form a magnetic path when the coil is energized so as to attract the plunger. The plunger has a small-diameter main body portion slidably guided in the tubular member and a large-diameter attracted portion having a surface facing the attracting member. The large-diameter attracted portion is located between the main body portion and the attracting member. The attracting member has a tubular projection provided on a surface thereof that faces the attracted portion of the plunger so that the attracted portion enters or withdraws from the tubular projection.
Thus, according to the present invention, the main body portion of the plunger is reduced in diameter, and the small-diameter main body portion is slidably guided by the tubular member that forms a magnetic path. Therefore, the magnetic flux density can be increased satisfactorily. In addition, because the attracted portion is made larger in diameter than the main body portion, it is possible to increase the area of the mutually opposing surfaces of the attracted portion and the attracting member that forms a magnetic path. As a result, it is possible to increase the magnetic force for attracting the plunger. In addition, a tubular projection is provided on the surface of the attracting member that faces the attracted portion of the plunger so that the attracted portion enters or withdraws from the tubular projection. Therefore, the tubular projection radially absorbs the magnetic flux from the plunger. Accordingly, it is possible to obtain attraction force proportional to the magnitude of the electric current supplied to the coil. Hence, it is possible to reliably change damping force characteristics in proportion to the magnitude of the electric current supplied to the coil.
Preferably, the plunger in the above-described damping force control type hydraulic shock absorber is inversely tapered so as to gradually increase in diameter from the main body portion toward the attracted portion.
With this arrangement, it is possible to smoothen the flow of magnetic flux between the main body portion and the attracted portion of the plunger and to spread the magnetic flux surely as far as the outermost periphery of the attracted portion. Accordingly, attraction force can be further increased, and the consumed current can be reduced. Consequently, it is possible to suppress the generation of heat from the coil and so forth and to change damping force characteristics stably.
According to one aspect of the invention, the damping force control type hydraulic shock absorber further includes a first bearing situated in the tubular member so that the plunger is slidably guided by the first bearing and a second bearing situated in the attracting member. The solenoid-operated driving mechanism includes an actuating rod having opposite end portions. One end portion of the actuating rod is connected to an end portion of the plunger on the side of the attracting member and the other end portion of the actuating rod extends through the attracting member and is slidably guided by the second bearing.
With this arrangement, the opposing surfaces of the attracted portion of the plunger and the attracting member are positioned between the first and second bearings. Therefore, it is possible to stabilize the radial position in which the plunger engages with the attracting member when the former is attracted by the latter.
According to another aspect of the invention, the tubular member has a bottom wall so that a space filled with a hydraulic fluid is defined by the tubular member and the plunger between the bottom and the plunger and wherein the plunger has an orifice passage axially extending through the plunger and having an orifice.
With this arrangement, abrupt movement of the plunger upon energizing the coil is damped.
The present invention a so provides a solenoid-operated driving apparatus comprising a coil provided in a casing, a tubular member provided in the coil at one end portion of the coil to form a portion of a magnetic path for the magnetic flux created when the coil is energized, a plunger slidably guided in the tubular member, and an attracting member provided in the coil at the other end portion of the coil to form another portion of the magnetic path when the coil is energized so as to attract the plunger. The plunger has a small-diameter main body portion slidably guided in the tubular member and a large-diameter attracted portion having a surface facing the attracting member. The large-diameter attracted portion is located between the main body portion and the attracting member. The attracting member has a tubular projection provided on a surface thereof that faces the attracted portion of the plunger so that the attracted portion enters or withdraws from the tubular projection.
Thus, also in the arrangement stated just above, the main body portion of the plunger is reduced in diameter, and the small-diameter main body portion is slidably guided by the tubular member that forms a magnetic path. Therefore, the magnetic flux density can be increased satisfactorily. In addition, because the attracted portion is made larger in diameter than the main body portion, it is possible to increase the area of the mutually opposing surfaces of the attracted portion and the attracting member that forms a magnetic path. As a result, it is possible to increase the magnetic force for attracting the plunger. In addition, a tubular projection is provided on the surface of the attracting member that faces the attracted portion of the plunger so that the attracted portion enters or withdraws from the tubular projection. Therefore, the tubular projection radially absorbs the magnetic flux from the plunger. Accordingly, it is possible to obtain attraction force proportional to the magnitude of the electric current supplied to the coil and hence possible to reliably change damping force characteristics in proportion to the magnitude of the electric current supplied to the coil.